Dual enhanced tube for vapor generator

ABSTRACT

A heat exchange assembly for generating steam from a second medium to be used to drive a steam turbine for generating electricity or for other process, includes a tube having a longitudinal axis and an inner wall and an outer wall. The outer wall includes a plurality of spaced fins oriented generally perpendicular to the longitudinal axis. The inner wall defines a preheat zone and a dual phase zone. The preheat zone defines a helical rib configured to provide swirling motion and increase heat transfer surface area to liquid entering the tube increasing heat transfer from the tube to the liquid. The dual phase zone is spaced from the preheat zone and defines a helical rib configured to provide swirling motion to steam and liquid passing through the dual phase zone increasing heat transfer from the tube to the steam and liquid while preventing vapor stagnation and film boiling.

BACKGROUND OF THE INVENTION

The use of steam, for driving steam turbines for the purpose of generating electricity or for other processes such as desalination or enhanced mineral recovery, has been a common practice for many years. Various methods of generating steam have been employed making use of fossil fuels, nuclear fusion, and more recently, using solar energy. Generally, a heat exchanger having a primary liquid that is heated from the various sources set forth above heats a secondary liquid to generate steam.

As the demands for steam for generating electricity and other recovery processes increase, it has become necessary to improve the heat transfer efficiency between primary and secondary mediums passing through a heat exchanger. Additionally, for improved solar plant economics, the immediate use of available solar energy has become very important, requiring optimized transfer of heat from the primary fluid to the secondary fluid during periods of solar field start-up and normal operations. During the start up when the primary fluid is being heated to a temperature reaching the desirable range, and mid-day operation when the primary fluid may be heated to a temperature a bit above the optimum range it is important to maintain optimum heat transfer between the two fluids.

Known steam generating heat exchangers have made use of smooth tubes as a part of a shell and tube heat exchanger where the primary heating medium flows on the outside of the tubes and the secondary vapor generating medium flows on the inside of the tubes. When the temperature of the primary medium is maintained within a narrow window, the smooth tubes have been known to provide satisfactory heat transfer. However, vapor blanketing inside the tubes is known to occur in the transition phase tube where the two phase flow with high vapor fraction flows over hot metal surface resulting in scaling of deposits known to reduce the heat transfer efficiency of the heat exchanger. Further problems arise when the temperature of the primary medium cannot be controlled within a narrow range, which is known to occur in solar fields. When a solar field heats the primary medium to a temperature above optimum, scaling occurs inside the transfer tubes at an advanced rate due to the rapid transition of the phase of the secondary fluid from liquid to vapor. During start ups, when the primary medium temperature is still less than optimum, heat transfer is very poor in parts of the heat exchanger and it takes a long period for generating required vapor fraction in the heat exchanger tubes. Furthermore, prior art methods of generating steam have intentionally restricted the amount of steam generation to about 10 to 12% of the amount of liquid passing through a heat exchanger to avoid the problems set forth above.

Therefore, to meet the demands of high-efficiency steam generation processes, it has become necessary to improve the heat transfer efficiency of the steam generating system and the ability to generate higher percentages of steam to liquid. Furthermore, it is also necessary to reduce the frequency of downtime during which the steam generating system is not operating due to the required cleaning of the inside of the steam generator tubes and by reducing the period of time for requisite cleaning of the tubes.

SUMMARY OF THE INVENTION

A heat exchanger assembly is used to transfer heat from a first medium to a second medium to convert the second medium from liquid to vapor. The vapor, or steam, is used to drive a steam turbine for generating electrical energy or for other processes such as desalination or mineral recovery. A tube is disposed inside the heat exchange assembly and includes an inner wall and an outer wall the length of which define a longitudinal axis. The outer wall includes a plurality of spaced fins oriented in a generally perpendicular manner to the longitudinal axis of the tube. The inner wall defines a preheat zone and a dual phase zone. The preheat zone defines a helical rib configured to provide swirling motion and increased heat transfer surface area to liquid entering the tube. The dual phase zone is spaced from the preheat zone and defines a helical rib configured to provide a swirling motion to the steam and liquid passing through the dual phase zone. In each instance, the helical rib improves the efficiency of heat transfer from the first medium to the second medium to convert liquid to steam.

The configuration of the inventive heat exchanger and dual enhanced tube is believed to provide heat exchange benefit for the specific purpose of converting liquid to vapor while meeting the demands of high-efficiency solar heat collector assemblies. By providing helical ribbing to the entry of a phase conversion tube, the efficiency of the first zone of the tube is improved to provide enhanced heat transfer from the first medium to the second medium while the second medium is still primarily in the liquid phase. In the zone before the outlet of the tube, where the second medium consists of high vapor fraction, the helical rib provides swirling turbulent motion to the vapor preventing vapor blanketing on the tube wall, which is known to cause reduced heat transfer as well as rapid scaling and fouling with deposits further resulting in the decrease of the performance of the heat exchanger. The dual enhanced tube provides the ability to increase the steam to liquid percentage to 20% without causing scaling inside the tube. This increased percentage over prior art tubes provides significant efficiency benefits. Therefore, the dual enhanced tube not only improves heat transfer efficiency, but also reduces the amount of maintenance and cleaning required of known steam generating systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 shows a cross-sectional view of a shell and tube heat exchanger having the inventive dual enhanced tube;

FIG. 2 shows a perspective view of the inventive dual enhanced tube;

FIG. 3 shows a cross-sectional view of the inventive dual enhanced tube with geometric identifiers

FIG. 3A shows a cross-sectional view of a first embodiment of the inventive dual enhanced tube;

FIG. 3B shows a cross-sectional view of an alternate embodiment of the inventive dual enhanced tube;

FIG. 4 shows an alternative embodiment of the shell and tube heat exchanger where the inventive dual enhanced tubes are bent into a generally u-shaped configuration; and

FIG. 4A shows the alternate embodiment of the dual enhanced tube.

DETAILED DESCRIPTION OF THE INVENTION

A heat exchanger assembly of the present invention is generally shown at 10 of FIG. 1. A first, or primary heating medium, enters the assembly 10 through heating medium inlet 12 and exits the assembly 10 through a heating medium outlet 14. The heating medium is contemplated by the inventors to be any liquid medium that is heated by an external energy source, for example, fossil fuel burning furnaces, nuclear energy reactors or solar energy collector fields. It is also contemplated by the inventors that the heating medium does not change phase, but remains in a liquid state throughout the process.

The second medium, or liquid, enters the assembly 10 through a liquid inlet 16 and exits the assembly 10 through a two phase flow outlet 18 after having been converted partially to vapor. It should be understood by those of skill in the art that the two phase flow will exit the assembly through the outlet 18. The second medium contemplated by the inventors is water that is converted to steam in the assembly 10 to be delivered into an external steam drum either by natural circulation or forced circulation by pumps. It should be understood by those of skill in the art that other mediums may be used, such as, for example, an ammonia water mixture or the like. Steam from the steam drum is then superheated in a heat exchanger and supplied to drive a steam turbine (not shown) generating electrical energy or supply to processes such as desalination or mineral recovery, in a known manner.

As best represented in FIG. 1, an inlet tube sheet 20 is located proximate the liquid inlet 16 and an outlet tube sheet 22 is located proximate the vapor outlet 18. A plurality of dual enhanced tubes 24 extend between the inlet tube sheet 20 and the outlet tube sheet 22 creating a heat transfer chamber 26 defined by an exterior of the dual enhanced tubes 24, an assembly housing or shell 28, and the inlet and outlet tube sheets 20, 22. The descriptive term, “phase transition” when associated with the dual enhanced tubes 24 is used in reference to the phase transition that occurs to the second medium between liquid and vapor. The dual enhanced tubes 24 are supported by a plurality of spaced baffle plates 25. As should be understood by one of ordinary skill in the art, the heating medium flows into the assembly 10 through the heating medium inlet 12, through the heat transfer chamber 26, and out of the heating medium outlet 14. The segmentally cut baffle plates 25 cause the heating medium to flow along a serpentine path to maximize the contact between the heating medium and the dual enhanced tubes 24. Also, the liquid flows through liquid inlet 16 into an inlet chamber 30 through dual enhanced tubes 24 to the outlet chamber 32 and subsequently through vapor outlet 18. The assembly 10 is sealed on opposing ends by endplates 34 a first of which defines the inlet chamber 30 with the inlet tube sheet 20, and a second of which defines the vapor chamber 32 with the outlet tube sheet 22.

FIGS. 2 and 3 show a perspective view and a cross-sectional view respectively of the dual enhanced tube of the present invention at 24. The dual enhanced tube 24 is defined by a substantially annular wall 36. The annular wall 36 includes an outer surface 38 and inner surface 40. The outer surface 38 includes a plurality of spaced fins 42 that extend radially outwardly from the annular wall 36. The fins 42 are contemplated by the inventors to be perpendicular to an axis defined by the length of the dual enhanced tube 24. However, it should be understood by those of skill in the art that the fins 42 can take an alternative orientation such as helical, or angled to the axis defined by the length of the dual enhanced tube 24.

Referring to FIGS. 3, 3A and 3B, substantially helical ribs 44 extend from the inner surface 40 of the annular wall 36. The helical ribs 44 have a first configuration at the inlet of the dual enhanced tube 24 where the second medium is in liquid phase. The helical ribs 44 are contemplated to take a second configuration at the exit of the dual enhanced tube 24 where, primarily, two phase flow exits the tube as shown in FIG. 3B. It is further contemplated by the inventor that an inner section spaced between the ends of a dual enhanced tube 24 includes a smooth inner surface 40 for purpose of which will be explained further below. However, it is also contemplated by the inventor that the helical ribs 44 defined on the inner surface 40 of the dual enhanced tube 24 is consistent from the inlet to the outlet of the dual enhanced tube 24 as shown if FIG. 3A.

The dimensional aspects of the dual enhanced tube are shown in FIG. 3. The height (do−dr)/2 of the external fins 42 is between about 0.6 mm and 0.7 mm having a target height of about 0.66 mm. The average external fin 42 thickness is between about 0.28 mm and 0.32 mm, with a target thickness of 0.3 mm. The fins 42 may also take a pyramidal shape where the fin thickness 42 is less at its apex than at its base. The fins also have a spacing, Wp, defined by the number of fins per unit length of tube, for example, between about 7 and 12 per centimeter length of tube.

Internally, the helical rib 44 has an average rib height (di−dp)/2 of between about 0.5 mm to 1.0 mm. The average thickness of the internal helical ribs 44 is between about 0.38 mm and 0.44 mm. The spacing, Ws of the helical ribs 44 at the inlet of the dual enhanced tube, as defined by the number of ribs per unit length of the tube is about 6 per centimeter length of tube at the inlet and 2 per centimeter length of tube at the outlet. The helical ribs 44 terminate in height at an apex having an apex angle of about ten to thirty degrees while along the axis of the tube forming a helix angle of ten to forty five degrees with respect to the tube axis.

During operation, liquid entering the dual enhanced tube 24 is initially heated in a first zone 46 which extends the length required to raise the temperature of a liquid to its boiling point. This is believed to be between about 15% and 33% of the length of the dual enhanced tube 24. Therefore, the actual length of the first zone 46 could vary depending upon the temperature of the heating medium. Furthermore, the length of the first zone 46 is shortened by the inventive dual enhanced tube due to the swirling turbulence generated by the helical ribs 44 and the improved heat transfer between mediums by the extended surface area generated by the outer fins 42. By shortening the length of the first zone 46, the phase transition zones of the dual enhanced tube 24 are lengthened providing enhanced phase transition to the liquid entering the dual enhanced tube 24.

A second zone 48 is contemplated to be in the central region of the dual enhanced tube 24 where the liquid, now raised to a temperature of its boiling point begins a transition from liquid to steam, or vapor. The flow velocity of the second medium increases in the second zone 48 due to a lower density attributable to the generation of some vapor. The lower density is believed to cause flow turbulence in the middle zone. Therefore, it is not believed that a substantial amount of vapor film is built up in this middle zone 48 that would cause scaling reducing heat transfer efficiency of the assembly 10. Therefore, a reduced cost dual enhanced tube is contemplated to have a smooth inner surface in the central region or middle zone while helical ribbing 44 is only swaged at the opposing ends of the dual enhanced tube 24.

A third zone 50 is located at the opposite end of the dual enhanced tube 24 from the first zone 46. The third zone 50 is a location in which more liquid is converted to vapor, or steam, known to result in vapor film coating the inner surface 40 of the dual enhanced tube 24. As set forth above, vapor film is known to reduce the heat transfer efficiency of the tube and also results in a build-up of deposits requiring frequent cleaning of the assembly 10. The helical ribbing provides swirling and turbulent motion to the vapor preventing a vapor film from occurring and subsequent build-up of deposits on the inner surface 40.

An alternative embodiment is shown in FIG. 4 where an alternative dual enhanced tube 124 is shown in a u-shaped configuration. Each of the elements that are common to the first embodiment shown in FIG. 1 is numbered in the 100 series in FIG. 4. In the alternative embodiment, the assembly 110 includes a heating medium inlet 112 a heating medium outlet 114, a liquid inlet 116 and a two phase flow outlet 118, the fluids of which are segregated by tube sheet 120. The heating medium circulates through heat transfer chamber or shell 126 and around segmentally cut baffle plates 127 to provide heat to the dual enhanced tube 124 as explained above. Liquid enters the assembly 110 through liquid inlet 116 into inlet chamber 130 where the liquid enters the dual enhanced tube 124 and the two phase flow exits the dual enhanced tube 124 passing into an outlet chamber 132.

An alternate dual enhanced tube 124 is shown in FIG. 4A. It should be understood by those of skill in the art that a plurality of u-shaped dual enhanced tubes 124 could be used in a commercialized embodiment. A single endplate 134 defines the inlet chamber 130 and the vapor chamber 132 with tube sheet 120, the chambers of which 130, 132 are separated by separation member 135. The alternative assembly 110 provides the benefit of the inventive dual enhanced tube 124 while reducing the amount of space necessary for adequately converting liquid to vapor. It is anticipated that the alternate dual enhanced tube 124 includes three zones, a first zone 146, where the first medium is primarily in the liquid phase, a second zone or middle zone 148, where steam or vapor begins to emerge from the liquid phase, and a third zone 150, where a substantive amount of steam or vapor exists with liquid as set forth above. Also as set forth above, the first zone and the third zone include a helical rib 144, the configuration of which might differ between zones to maximize phase transition efficiency. Further, the second or middle zone 148 may have a smooth inner surface, or may include a helical rib 144.

While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A heat exchange assembly for generating steam from a second medium comprising: a tube defining a longitudinal axis and having an inner wall and an outer wall, said outer wall including a plurality of spaced fins oriented generally perpendicular to said longitudinal axis of said tube; said inner wall defining a preheat zone and a dual phase zone, said preheat zone defining a helical rib configured to provide swirling motion and increase heat transfer surface area to liquid entering said tube thereby increasing heat transfer from said tube to said liquid, and said dual phase zone being spaced from said preheat zone and defining a helical rib configured to provide swirling motion to steam and liquid passing through said dual phase zone thereby increasing heat transfer from said tube to said steam and liquid.
 2. The assembly set forth in claim 1, wherein said assembly includes a plurality of tubes configured in a bundle, each tube converting liquid to steam.
 3. The assembly set forth in claim 1, wherein said inner wall includes a smooth wall section separating said preheat zone and said dual phase zone.
 4. The assembly set forth in claim 1, wherein said spaced fins disposed upon said outer wall of said tube includes an average height of between about 0.6 mm and 0.7 mm.
 5. The assembly set forth in claim 1, wherein said spaced fins disposed upon said outer wall of said tube includes an average height of about 0.66 mm.
 6. The assembly set forth in claim 1, wherein said spaced fins disposed upon said outer wall of said tube include an average fin thickens of between about 0.2 mm and 0.4 mm.
 7. The assembly set forth in claim 1, wherein said spaced fins disposed upon said outer wall of said tube includes an average thickness of about 0.3 mm.
 8. The assembly set forth in claim 1, wherein said spaced fins include a density of between about 7 to 12 per centimeter.
 9. The assembly set forth in claim 1, wherein said rib includes an average height between about 0.5 mm and 1.0 mm.
 10. The assembly set forth in claim 1, wherein said rib includes an average rib thickness of between about 0.38 mm and 0.44 mm.
 11. The assembly set forth in claim 1, wherein said helical rib includes an apex angle with respect to tube radius of between about zero and thirty degrees.
 12. The assembly set forth in claim 1, wherein said helical rib includes a helix angle to said tube axis of between about ten and forty five degrees.
 13. A method of converting liquid to vapor using a heating fluid, comprising the steps of: providing a tube having a first zone, a second zone, and a third zone defined by an inner surface of said tube; heating an exterior surface of said tube with the heating fluid; preheating the liquid passing through said tube in said first zone while providing swirling motion the liquid thereby improving heat transfer from said tube to the liquid; converting at least a portion of the liquid to vapor in said second zone; and providing swirling motion to the vapor and liquid passing through said third zone thereby preventing film boiling from occurring on the surface of said third zone.
 14. The method set forth in claim 13, wherein said step of providing swirling motion to the liquid passing through said first zone is further defined by providing a helical rib to said inner surface of said tube at said first zone.
 15. The method set forth in claim 13, wherein said step of providing a tube is further defined by providing a tube with a second zone having a substantially smooth surface.
 16. The method set forth in claim 13, wherein said step of providing a tube is further defined by providing a tube having a two phase zone comprising vapor and liquid.
 17. The method set forth in claim 13, further including the step of providing sufficient heat energy to said third zone from the heating fluid to continuously maintain both vapor and liquid inside said tube.
 18. The method set forth in claim 13, wherein said step of providing swirling motion to the liquid passing through said third zone is further defined by providing a helical rib to said inner surface of said tube at said first zone.
 19. The method set forth in claim 13, wherein said step of providing a tube having a first zone, a second zone, and a third zone is further defined by providing a tube having a first zone comprising about 20% of a length of said tube, a second zone comprising about 60% of a length of said tube and a third zone comprising about 20% of a length of said tube.
 20. The method set forth in claim 13, further including the step of providing a different swirling motion to the liquid passing through said first zone than the swirling motion provided to the vapor and liquid passing through said third zone.
 21. The method set forth in claim 13, further including the step of providing a helical rib to said inner surface of said tube thereby increasing the surface area of said inner surface of said tube providing improved heat transfer from the heating fluid to the liquid passing through said tube.
 22. The method set forth in claim 21, wherein said step of providing a helical rib to said inner surface of said tube is further defined by providing a helical rib to said first and said second zones.
 23. A heat exchange assembly for converting water to steam, comprising: a housing having an inlet and an outlet; a plurality of baffle plates oriented in a predetermined fixed relationship, spaced apart creating a serpentine path inside said housing providing a route for a heating medium flowing from said inlet to said outlet; a plurality of tubes defining an inner wall and an outer wall, said tubes being disposed in said housing in a substantially vertical orientation, each of said plurality of tubes being received by at least some of said baffle plates providing contact to said outer wall with the heating medium flowing along said serpentine path; said outer wall of said tubes defining fins thereby increasing the surface area of said outer wall and said inner wall defining helical ribbing, said helical ribbing providing swirling motion to water entering said tube and swirling motion to water and steam passing through said tube thereby reducing film boiling associated with converting water to steam while increasing heat transfer through increased surface area of said inner wall associated with said helical ribbing.
 24. The assembly set forth in claim 23, wherein said housing includes an inlet plenum for delivering water to said plurality of tubes and outlet plenum for evacuating steam from said plurality of tubes.
 25. The assembly set forth in claim 23, wherein said inner wall of said tubes include a first zone, a second zone, and a third zone, said first zone being positioned adjacent said inlet plenum, said third zone being positioned adjacent said outlet plenum and said second zone being positioned between said first zone and said third zone.
 26. The assembly set forth in claim 25, wherein said first, second, and third zones each include helical ribbing thereby providing swirling motion to the water and the steam passing through said plurality of tubes.
 27. The assembly set forth in claim 25, wherein said second zone comprises a substantially smooth surface of said inner wall and said first zone and said third zone comprise helical ribbing for providing swirling motion to the water adjacent said inlet plenum and swirling motion to the steam adjacent said exit plenum.
 28. The assembly set forth in claim 23, wherein said fins are oriented in a substantially perpendicular relationship to the orientation of said tubes.
 29. The assembly set forth in claim 23, wherein said fins disposed upon said outer wall of said tube includes an average height of between about 0.6 mm and 0.7 mm.
 30. The assembly set forth in claim 23, wherein said fins disposed upon said outer wall of said tube includes an average height of about 0.66 mm.
 31. The assembly set forth in claim 23, wherein said fins disposed upon said outer wall of said tube include an average fin thickens of between about 0.2 mm and 0.4 mm.
 32. The assembly set forth in claim 23, wherein said fins disposed upon said outer wall of said tube includes an average thickness of about 0.3 mm.
 33. The assembly set forth in claim 23, wherein said fins include a density of between about 7 to 12 per centimeter.
 34. The assembly set forth in claim 23, wherein said helical ribbing includes an average height between about 0.5 mm and 1.0 mm.
 35. The assembly set forth in claim 23, wherein said helical ribbing includes an average rib thickness of between about 0.38 mm and 0.44 mm.
 36. The assembly set forth in claim 23, wherein said helical ribbing defines an apex angle relative to a tube radius of between about zero and thirty degrees.
 37. The assembly set forth in claim 23, wherein said helical ribbing includes a helix angle to said tube axis of between about ten and forty five degrees. 